The present invention relates generally to improved methods for the microbial production of acetic acid. More particularly, the invention relates to extraction of acetic acid from aqueous streams, and from the microbial fermentation of desirable chemical products from gaseous streams, such as waste gas streams, industrial gas streams, or from gas streams produced from the gasification of carbonaceous materials.
Methods for the anaerobic fermentation of carbon monoxide, and/or hydrogen and carbon dioxide to produce acetic acid, acetate salts or other products of commercial interest, such as ethanol, have been performed at laboratory bench scale. See, e.g., Vega et al, (1989) Biotech. Bioeng., 34:785-793; Klasson et al (1990) Appl. Biochem. Biotech., 24/25: 1; Vega et al (1989) Appl. Biochem. Biotech., 20/21: 781-797; and Klasson et al (1992) Enz. Microbio. Tech. 19: 602-608, among others. More recently, the present inventors have discussed large-scale methods for the fermentation of industrial gas streams, particularly waste gas streams, into products of commercial use by using methods employing fermentation of the gas stream, an aqueous nutrient medium and an anaerobic bacteria or mixtures thereof in a bioreactor. See, e.g., U.S. Pat. No. 5,173,429; U.S. Pat. No. 5,593,886 and International Patent Publication No. WO98/00558, incorporated herein by reference.
According to the above-cited prior art of the inventors, one such large scale process involves the following summarized steps. Nutrients are continuously fed to a bioreactor or fermenter in which resides a culture, either single or mixed species, of anaerobic bacteria. A gas stream is continuously introduced into the bioreactor and retained in the bioreactor for sufficient time to maximize efficiency of the process. Exhaust gas containing inert and unreacted substrate gases, are then released. The liquid effluent is passed to a centrifuge, hollow fiber membrane, or other solid-liquid separation device to separate out microorganisms that are entrained. These microorganisms are returned to the bioreactor to maintain a high cell concentration which yields a faster reaction rate. Separation of the desired biologically produced product(s) from the permeate or centrifugate occurs by passing the permeate or centrifugate to an extractor where it is contacted with a solvent, such as a di-alkyl and tri-alkyl amine in a suitable cosolvent, or tributyl phosphate, ethyl acetate, tri-octyl phosphine oxide and related compounds in a co-solvent. Suitable cosolvents include long chain alcohols, hexane, cyclohexane, chloroform, and tetrachloroethylene.
The nutrients and materials in the aqueous phase pass back to the bioreactor and the solvent/acid/water solution passes to a distillation column, where this solution is heated to a sufficient temperature to separate the acid and water from the solvent. The solvent passes from the distillation column through a cooling chamber to lower the temperature to the optimum temperature for extraction, then back to the extractor for reuse. The acid and water solution passes to a final distillation column where the desired end product is separated from the water and removed. The water is recirculated for nutrient preparation.
Further, a variety of acetogenic bacteria are well known to produce acetic acid and other commercially interesting products when subjected to such fermentation processes, including novel strains of Clostridium ljungdahlii [See, e.g., U.S. Pat. Nos. 5,173,429 and 5,593,886 and International Patent Publication No. WO98/00558].
Despite such knowledge and advances in the art of microbial fermentation of a variety of gas streams, acetic acid production is limited by the acetic acid loading potential of the solvent used, and by the degradation of the solvent as it travels through the production process, among other issues. In view of the ever-increasing need to produce acetic acid, as well as to convert industrial waste gases into useful non-polluting products, there remains a need in the art for processes which are more efficient in producing the desired commercial product and compositions which can enhance performance of such methods.
In one aspect, the present invention provides a modified water-immiscible solvent useful in the extraction of acetic acid from aqueous streams comprising a substantially pure mixture of isomers of highly branched di-alkyl (or secondary) amines. This solvent can extract the acid in the absence of a co-solvent. In a preferred embodiment, this solvent is a modified form of Adogen283(copyright) solvent [Witco Corp.] which is substantially reduced in its content of alcohols and monoalkyl (or primary) amines. In another preferred embodiment, the solvent is further reduced in content (i.e. substantially purified) of tri-alkyl (or tertiary) amines.
In another aspect, the invention provides a method for treating a solvent comprising alcohols, monoalkyl amines, a mixture of isomers of highly branched di-alkyl amines and tri-alkyl amines to improve its acetic acid extractive capacity comprising distilling from the solvent substantially all the alcohols and monoalkyl amines. In another embodiment, the method involves subjecting the distilled solvent to a second distillation to remove substantially all tri-alkyl amines.
In yet a further aspect, the invention provides a novel water-immiscible solvent/co-solvent mixture useful for the extraction of acetic acid, preferably at concentrations less than 10%, from an aqueous stream comprising an above-described modified water-immiscible solvent useful in the extraction of acetic acid from aqueous streams comprising a substantially pure mixture of isomers of highly branched di-alkyl amines and a selected cosolvent. In a preferred embodiment, the cosolvent is a low boiling hydrocarbon having from 9 to 11 carbon atoms, which hydrocarbon forms an azeotrope with water and acetic acid.
In still another aspect, the invention provides a non-fermenting process for obtaining acetic acid from an aqueous stream comprising contacting the stream with a modified solvent/cosolvent mixture as described above; extracting the acetic acid from the aqueous phase into the solvent phase; and distilling the acetic acid from its admixture with the solvent under a temperature not exceeding 160xc2x0 C.
In yet a further aspect, the invention provides a non-fermenting process for obtaining acetic acid from an aqueous stream comprising contacting the stream with a solvent/cosolvent mixture as described above; extracting the acetic acid from the aqueous phase into the solvent/cosolvent phase; and distilling the acetic acid from its admixture with the solvent/cosolvent under a temperature not exceeding 160xc2x0 C. under a vacuum.
In a further aspect, the present invention provides an anaerobic microbial fermentation process for the production of acetic acid, the process comprising the steps of (a) fermenting in a bioreactor an aqueous stream comprising a gas selected from the group consisting of carbon monoxide, carbon monoxide and hydrogen, hydrogen and carbon dioxide, and carbon monoxide, carbon dioxide and hydrogen, in a nutrient mixture with an anaerobic acetogenic bacterium, thereby producing a broth comprising acetic acid; (b) continuously extracting acetic acid from the broth with a modified solvent/cosolvent mixture as described above; (c) continuously distilling from the product of (b) the acetic acid separately from the solvent at a temperature not exceeding 160xc2x0 C., and (d) optionally recycling the solvent and the broth through the bioreactor. The extracting and distilling steps occur without substantially degrading the amine to an amide, thus enhancing the efficiency of acetic acid recovery from the broth.
In still another aspect, the present invention provides a method for enhancing the recovery of acetic acid from a fermentation broth comprising an aqueous stream containing one or more of carbon monoxide, carbon dioxide and hydrogen, and an anaerobic acetogenic bacterium, and nutrient medium, the method comprising contacting the stream with a solvent comprising the above-described modified di-alkyl amine and a selected co-solvent; continuously extracting the acetic acid from the stream in the solvent mixture, and distilling acetic acid from the solvent mixture, under a vacuum at a distillation temperature of below 160xc2x0 C. therefrom, without substantially degrading the amine to amide.
In still another aspect, the invention provides an improved method for enhancing the recovery of acetic acid from anaerobic microbial fermentation of an aqueous stream comprising carbon monoxide, carbon monoxide and hydrogen, carbon monoxide, carbon dioxide and hydrogen, or carbon dioxide and hydrogen, wherein the method comprises the steps of contacting the fermentation product of the stream with a water-immiscible solvent, extracting the fermentation product from the stream, and distilling acetic acid therefrom. The improvement comprises employing as the solvent the modified solvent/cosolvent mixture described above and performing the distillation step at a temperature not exceeding 160xc2x0 C. without substantially degrading the amine to amide.
In yet a further aspect, the invention provides an anaerobic microbial fermentation process (i.e., an extractive fermentation process) for the production of acetic acid which is accomplished without filtration or cell separation occurring prior to extraction. In one embodiment, this method involves providing in a fermenter an anaerobic acetogenic bacterium in a nutrient mixture and a modified water-immiscible solvent comprising a substantially pure mixture of isomers of highly branched di-alkyl amines with a selected cosolvent, for a time sufficient to acclimate the bacteria to the solvent. Into the fermenter is introduced a gas stream comprising one or more of carbon dioxide, carbon monoxide and hydrogen and a fermentation broth comprising the bacteria, nutrient medium, acetic acid, solvent mixture and water. The fermentation broth containing the cells and solvent mixture is introduced into a settling tank, wherein an aqueous phase containing the bacteria and nutrient medium settles to the bottom of the tank from the solvent phase which contains acetic acid, solvent and water, without filtration. Continuous distillation under a temperature not exceeding 160xc2x0 C. removes the acetic acid separately from the solvent. The distilling step occurs without substantially degrading the amine to an amide, thus enhancing the efficiency of acetic acid recovery from the broth.
In still another aspect, the invention provides an anaerobic microbial fermentation process (i.e., a direct contact extraction process) for the production of acetic acid, which involves no filtration of bacterial cells. The process comprises the steps of: (a) fermenting in a bioreactor an aqueous stream comprising a gas containing one or more of carbon monoxide, carbon dioxide and hydrogen in a nutrient mixture with an anaerobic acetogenic bacteria, thereby producing a broth comprising acetic acid, water, and bacterial cells; (b) introducing into a conventional extraction device, such as a column with either solvent or water as the continuous phase (i) the broth with no cell separation and (ii) a solvent mixture comprising a modified water-immiscible solvent useful in the extraction of acetic acid from aqueous streams comprising a substantially pure mixture of isomers of highly branched di-alkyl amines and a selected co-solvent, wherein a solvent phase containing acetic acid, solvent and water exits the column separately from an aqueous phase comprising the bacteria and nutrient media; and (c) continuously distilling from the solvent phase of (b) the acetic acid separately from the solvent at a temperature not exceeding 160xc2x0 C. The steps (b) and (c) occur without substantially degrading the amine to an amide, thus enhancing the efficiency of acetic acid recovery from the broth.
In still another aspect, the invention provides an anaerobic microbial fermentation process for the production of acetic acid which comprises the step(s) of removing dissolved carbon dioxide, and optionally dissolved hydrogen sulfide, from the fermentation broth before extraction. The steps of this process can include (a) fermenting in a bioreactor a gas stream comprising one or more of carbon monoxide, carbon dioxide and hydrogen in a nutrient mixture with an anaerobic acetogenic bacteria, thereby producing a fermentation broth comprising acetic acid and dissolved carbon dioxide; (b) removing the carbon dioxide from the fermentation broth prior to extraction; (c) contacting the broth (b) with a solvent containing a di-alkyl amine, preferably the modified solvent/cosolvent mixture of this invention for a time sufficient to cause the formation of a solvent phase containing acetic acid, the solvent and water; and (d) continuously distilling acetic acid from the solvent phase. The carbon dioxide/hydrogen sulfide removal step may be accomplished with a stripping gas, by preheating the broth or by reducing the fermentation broth pressure rapidly.
Other aspects and advantages of the present invention are described further in the following detailed description of the preferred embodiments thereof.